The invention relates to a process for the separation of a hydroxylammonium salt solution from an aqueous reaction mixture comprising solid catalyst particles by means of filtration.
An important application of hydroxylammonium salts is the preparation of oximes from ketones or aldehydes, in particular the preparation of cyclohexanone oxime from cyclohexanone. For this way of preparation of an oxime a cyclic process is known wherein an acid-buffered reaction medium is kept in circulation via a hydroxylammonium salt synthesis zone and an oxime synthesis zone. The reaction medium is acid-buffered by means of for instance phosphoric acid and/or sulphuric acid and the buffer salts derived from these acids, for instance alkali and/or ammonium salts. In the hydroxylammonium salt synthesis zone, nitrate ions or nitrogen oxides, are converted with hydrogen to hydroxylamine. The hydroxylamine reacts with free buffer acid to produce the corresponding hydroxylammonium salt, which is subsequently led to the oxime synthesis zone where it reacts with a ketone to the corresponding oxime, with release of acid. After separation of the oxime from the reaction medium the reaction medium is recycled to the hydroxylammonium salt synthesis zone and fresh nitrate ions or nitrogen oxides are added, to the reaction medium.
In the case that the hydroxylammonium salt synthesis starts from a solution of phosphoric acid and nitrate the above-mentioned chemical reactions are represented as follows:
1) Preparation of the hydroxylammonium salt:
2H3PO4+NO3xe2x88x92+3H2xe2x86x92NH3OH++2H2PO4xe2x88x92+2H2O
2) Preparation of the oxime: 
3) Suppletion of fresh nitrate ions in the form of HNO3, after separation of the oxime formed:
H3PO4+H2PO4xe2x88x92+3H2O+HNO3xe2x86x922H3PO4+NO3xe2x88x92+3H2O
The first reaction is catalyzed heterogeneously. Preferably, the catalyst is present as finely divided solids as a disperse phase in a liquid reaction mixture.
The reaction mixture of the first step is an aqueous reaction mixture comprising a suspension of solid catalyst particles in a hydroxylammonium salt solution. Before this aqueous reaction mixture is transported to the oxime synthesis zone, the solid catalyst particles are preferably separated from this aqueous reaction mixture resulting in a hydroxylammonium salt solution. The catalyst applied in the preparation of the hydroxylammonium salt mostly consists of a metal from the platinum metal group, for instance Pd or Pd+Pt as active component on a carrier material such as for instance carbon. The catalyst may be activated by the presence of one or more catalyst activators. The catalyst activator may be an element from the group comprising Cu, Ag, Cd, Hg, Ga, In, TI, Ge, Sn, Pb, As, Sb and Bi. Compounds containing the elements in question may also be used as catalyst activators, for example oxides, nitrates, phosphates, sulphates, halogenides, and acetates. The elements or their compounds can be directly applied to the catalyst as described in U.S. Pat. No. 3,767,758 or they can be added to the reaction medium.
It is known from for example U.S. Pat. No. 5,792,439 that with a Pd+Pt on carbon catalyst a high level of activity and a reasonable degree of selectivity are achieved in the reaction of nitrate to hydroxylammonium salt. It is known from for example WO-A-9818717 that with a Pd on carbon catalyst a higher degree of selectivity in the hydroxylammonium salt synthesis is achieved.
EP-A-577213 describes a process for the separation of a hydroxylammonium salt solution from an aqueous reaction mixture containing solid catalyst particles by means of crossflow filtration.
A drawback of the use crossflow filtration according to EP-A-577213 is that the pores of the filter soon get clogged up by small catalyst particles, so that the flow through the filter is low and backwashing is often required. Consequently the filtration capacity is low.
The aim of the invention is to provide a filtration process with a higher capacity.
This aim is achieved in that cake filtration through an asymmetric filter is applied, the asymmetric filter comprising at least two layers having different pore diameter, at least one layer being a filter layer, the pore diameter of the filter layer being between 0.1 and 10 xcexcm and the thickness of the filter layer being between 10 to 1000 xcexcm.
Due to application of a process according to the invention in an existing installation for preparing hydroxylammonium salts, the capacity of the existing installation can be increased.
Cake filtration is a technique in which a layer of solid particles will be deposited on the filter material thereby producing a filter cake. The solid particles are thus retained by the filter, while a liquid phase passes through as filtrate. In the process according to the invention, a layer of catalyst (filter cake) will be deposited on the filter material. Effective functioning of a filter using cake filtration even requires the formation of a filter cake.
The asymmetric filter is composed of several layers and at least of two layers; the pore diameter of one layer is larger than the pore diameter of the other layer. At least one layer being a filter layer. The asymmetric filter will mostly be composed of two layers, one layer having a structural function (the structural layer) and the other layer constituting the actual filter (the filter layer). The pore diameter of the structural layer is larger than that of the filter layer. Where the term xe2x80x98pore diameterxe2x80x99 is used in this description, the pore diameter of the filter layer is always meant. The thickness of the filter layer is 10 to 1000 xcexcm. The thickness of the filter layer is preferably 100 to 500 xcexcm. The thickness of the filter layer of an asymmetric filter according to the invention is normally equal to 1 to of the wall thickness of the filter.
The pore diameter of the filter layer is between 0.1 and 10 xcexcm, preferably between 1 and 5 xcexcm.
The catalyst particles have in general an average size of between 5 and 150 xcexcm, preferably between 10 xcexcm and 60 xcexcm. By xe2x80x9caverage particle sizexe2x80x9d is meant that 50 vol % of the particles are larger than the specified diameter. In particular, within said diameter range and size range, respectively, the average size of the catalyst particles is larger than the average pore diameter of the filter layer.
The filter medium is in general a microporous substance such as for instance carbon, sintered metal, polymers and materials of ceramic origin, such as for instance Al2O3 en SiC. By preference the material of which the filter medium consists is SiC, carbon or sintered metal.
According to the state of the art, separation of solid catalyst particles from a hydroxylammonium salt solution can also be performed by cake the separation is carried out according to the process of claim 1 filtration through a normal filter with a thickness of 2-5 mm and a pore diameter of 0.5-50 xcexcm.
This involves a great pressure drop across the filter, which is in part due to the constitution of a thick filter cake. This filter also gets clogged by small catalyst particles.
In the process according to the invention, an asymmetric filter with a thinner filter layer is used resulting in a reduction of the rate and the intensity of clogging by small catalyst particles. The pressure drop across an asymmetric filter is lower than the pressure drop across a normal filter of the same overall thickness. It has surprisingly been found that when an asymmetric filter is used the pressure drop across the filter cake is also lower, so that it is possible to work with a much lower pressure drop than expected. An additional advantage of this is that owing to the low pressure drop the occurrence of filter rupture is reduced. Moreover, in comparison with a normal cake filtration, the filtration capacity increases when an asymmetric filter is used.
The temperature at which the filtration is carried out will in part depend on the temperature which prevails in the hydroxylammonium salt synthesis zone and in part on the thermal stability of the filter medium. The filtration is in general carried out at a temperature of 25-100xc2x0 C. and by preference at 45-65xc2x0 C.
The pressure at which the filtration is effected, as well as the pressure difference, will depend on the process pressure prevailing in the hydroxylammonium salt synthesis zone. The pressure to be applied will in general lie between 0 and 5 MPa. The pressure will preferably lie between 1 and 4 MPa. The pressure difference across the filter medium will depend on the filter medium and on the desired capacity of the filter. In general the pressure difference can be low due to the use of the asymmetric filters and will lie between 0 and 0.4 MPa. By preference a pressure difference between 0.02 and 0.07 MPa is applied.
The invention also relates to a process for the preparation and purification of a hydroxylammonium salt solution, wherein the hydroxylammonium salt solution is separated from an aqueous reaction mixture containing inter alia solid catalyst particles by means of the filtration process as described above. In one embodiment of this invention, the hydroxylammonium salt is filtered with the process as described above, part of the dissolved hydroxylammonium salt being separated off and the remaining aqueous reaction mixture, including the heterogeneous catalyst, being returned to the hydroxylammonium salt synthesis zone.
The catalyst used in the preparation of a hydroxylammonium salt solution preferably is a Pd on carbon or a Pd+Pt on carbon catalyst. The Pd on carbon catalyst in general has a Pd content of 1 to 25 wt. %, by preference 7 to 20 wt. %. The Pd+Pt on carbon catalyst in general has a (Pd+Pt) content of 1 to 20 wt. %, by preference 5 to 15 wt. %. The Pd:Pt weight ratio is in general between 1:1 and 10:1.
The catalyst concentration is in general between 0.001 and 0.1 gram of catalyst per gram of reaction mixture. The catalyst concentration is by preference between 0.005 and 0.05 gram of catalyst per gram of reaction mixture.
The invention will be further elucidated by means of the following examples without being limited thereto.